In recent years, a spread of transactions across the Internet has led to a rapid increase in information transmission amount. Also in other fields, there has been an increase in information transmission amount. With the increase in information transmission amount, there is a demand for large transmission capacity and high density of transmission systems. In view of the above, in a WDM transmission system (wavelength division multiplexing transmission system), the transmission band has been expanded from C-band (1530 to 1560 nm) to L-band (1560 to 1620 nm), and further, to S-band (1475 to 1510 nm). FIG. 6A shows an example of a WDM transmission system. In this WDM transmission system, lumped amplifiers 11 for respective bands are used in repeaters 10, each of which contains a pumping light source 12 for distributed amplification.
A WDM optical signal transmitted from a transmitter 13 of the WDM transmission system of FIG. 6A undergoes Raman amplification by optical fiber amplifiers 14 using a transmission line as the amplification medium, and optical amplification by the repeaters 10. In this case, the WDM optical signal input to each repeater 10 is demultiplexed by a demultiplexer 15, amplified by the optical amplifiers 11 which are contained in each repeater 10 and adapted to operate for the different bands, and multiplexed by a multiplexer 16 before being output. Prior to this amplification, the WDM optical signal undergoes Raman amplification by the optical fiber amplifiers 14 using the transmission line as the amplification medium by means of pumping lightwave from the pumping light source 12 for distributed amplification of the repeater 10.
When transmitting WDM optical signals of the three bands: S-band, C-band, and L-band, from the transmitter 13 of FIG. 6A, these optical signals are ideally output from the repeaters 10 without changing the optical level at which they are input to the transmission line; actually, however, a deviation in output level is generated due to the wavelength dependency of fiber loss, inter-signal Raman effect, and characteristics of the lumped amplifiers. In particular, the deficiency in power of the S-band signal is conspicuous. The inter-signal Raman effect means energy loss of short-wavelength light due to transferring the energy to the long-wavelength light via Raman amplification. The WDM optical signal is subjected to fiber loss as shown FIG. 7 when a WDM optical signal is transmitted through an optical fiber having a length of 80 km. FIG. 6B shows an example of an ideal WDM output at point O in FIG. 6A (in front of the repeater 10); FIG. 6C shows an example of a WDM output obtained by taking into account solely the fiber loss; and FIG. 6D shows an example of a WDM output obtained by taking into account solely the inter-signal Raman effect. Here, it is assumed that the lumped amplifier gain for S-band is smaller than those for C-band and L-band.
According to a three-band transmission method, the input level of the S-band optical signal is made higher than the input level in the other bands to thereby compensate for the deficiency in power of the S-band signal and equalize the output levels of the signals of the three bands. Apart from this, there is a method according to which a distributed Raman amplifier is used in the repeater, solely the S-band signal being amplified by the distributed Raman amplifier.
In a WDM transmission system using a distributed Raman amplifier in the repeater, designing the pumping wavelength with only the flatness at the wavelength band giving maximum gain (e.g., S-band) in mind may result in generation of a gain ripple A on a long-wavelength band side (i.e., C-band) with respect to the wavelength band for which the pumping wavelength is optimized, as shown in FIG. 8. The ripple A of FIG. 8 is at a wavelength of 1530 nm, which is the peak of gain obtained by pumping lightwave of a wavelength of 1413 nm. When an optical signal of a band (C-band) with the ripple A is used at the same time, the flatness of the signal level is adversely affected.